POLYISOBUTYLENE-CONTAINING BLOCK POLYMERS BY SEQUENTIAL MONOMER ADDITION .4. NEW TRIBLOCK THERMOPLASTIC ELASTOMERS COMPRISING HIGH TG STYRENIC GLASSY SEGMENTS - SYNTHESIS, CHARACTERIZATION AND PHYSICAL-PROPERTIES

New linear triblock thermoplastic elastomers (TPEs) comprising a rubbery polyisobutylene (PIB) midblock flanked by two glassy endblocks of various styrenic polymers have been synthesized by living carbocationic polymerization by sequential monomer addition. First isobutylene (IB) was polymerized by a bifunctional tert-ether (dicumyl methyl ether) initiator in conjunction with TiCl4 coinitiator in CH3Cl/methylcyclohexane (MeCHx) (40/60 v/v) solvent mixtures at -80-degrees-C. After the living narrow molecular weight distribution PIB midblock (M(n)BAR = 1.1-1.2) has reached the desired molecular weight, the styrenic monomers together with an electron pair donor (ED) and a proton trap (di-tert-butylpyridine, DtBP) were added to start the blocking of the glassy segments from the living + PIB + chain ends. While p-methylstyrene (pMeSt), p-t-butylstyrene (ptBuSt) and indene (In) gave essentially 100% blocking to the corresponding glassy endblocks, the blocking of 2,4,6-trimethylstyrene (TMeSt) and alpha-methylstyrene (alpha-MeSt) were ineffective. Uncontrolled initiation by protic impurities was prevented by the use of DtBP. In the simultaneous presence of DtBP and the strong ED N,N-dimethylacetamide (DMA), TPEs with good mechanical properties (10-20 MPa tensile strength, 300-600% elongation) were prepared. The products exhibit a low and a high temperature T(g) characteristic of phase separated rubbery and glassy domains. The service temperature of these new TPEs exceeds that of PSt-PIB-PSt triblock copolymers due to the higher T(g)s (PpMeSt = 108, PptBuSt = 142 and PIn = 220-240-degrees-C) of the outer blocks. The T(g) of the glassy blocks can be regulated by copolymerizing two styrene derivatives; a triblock copolymer with outer blocks of poly (p-t-butylstyrene-co-indene) showed a single glassy transition T(g) = +165-degrees-C, i.e., in between that of PptBuSt and PIn. Virgin TPEs have been repeatedly compression molded without deterioration of physical properties. The high melt flow index obtained with a TPE containing PptBuSt endblocks suggests superior processability relative to those with PSt endblocks. The tensile strength retention at 60-degrees-C of the former TPE is far superior to that of a PSt-PIB-PSt triblock of similar composition.